Recovery of isoprene from hydrocarbon fractions



RECOVERY OF ISOPRENE FROM HYDROCARBON FRACTIONS Filed Nov. 5. 1965 Jan.31, 1967 R. w. KING ETAL 3 Sheets-Sheet 2 w m mm 5 m mm .y wow \W 1? Mm#m 1 -\\l ww mN/EQ mm QM mom mm ww M535 mm QQJAQ/ :i @238 m QC 6m vow vi. M7 Q U FAWN v 5 53 1 2m @2326 mm. 8N l V \mN mm mwom w mm %m 6w N NONJan, 31, 1967 R. W. KING ETAL RECOVERY OF ISOPRENE FROM HYDROCARBONFRACTIONS Filed Nov. 5. 1965 3 Sheets-Sheet 3 8m 8m 2% m 4% 2&5 r ma l rwmm ER mm 8N mm C Km Em -E f bm 2 @258 i mm 1% km SN mm M53: 1 @238 U mm3w 7 X mm m@ 9%? 6w mw United States Patent (3 3,301,915 RECOVERY OFISUPRENE FROM HYDROCARBON FRACTIONS Ralph William King, London, andBarry Owen Shorthouse, Southampton, England, assignors to TheInternational Synthetic Rubber Company Limited, Southampton, EnglandFiled Nov. 5, 1965, Ser. No. 513,632 Claims priority, application GreatBritain, Nov. 6, 1964, 45,358/ 64 11 Claims. (Cl. 260-6815) Thisinvention relates to the recovery of isoprene from hydrocarbon streamsconsisting wholly or predominantly of C hydrocarbons and particularlyfrom such streams which contain in addition to isoprene appreciableamounts of cyclopentadiene and usually also of piperylenes.

Isoprene-containing hydrocarbon streams are available which are obtained(i) by the chemical or catalytic dehydrogenation of C olefin and/ orparafiin hydrocarbons,

'(ii) by thermal demethenation of specific C hydrocarbons such as 2methyl pentene 2, and (iii) as a by-product stream of the thermalcracking of naptha and other bydrocarbon feedstocks for the productionof ethylene. This by product stream forms a particularly suitablefeedstock for the process of the invention. In the last mentionedprocess other valuable unsaturated gaseous hydrocarbons (propylene,butadiene) are produced simultaneously and usually separated andrecovered.

The liquid product from the cracking operations referred to above isnormally separated into two or more fractions of different oroverlapping boiling range, the main fraction consisting of raw gasolinewith a boiling range of approximately to 150 C. This main fractioncontains appreciable amounts of C hydrocarbons, both cyclic and acyclicparaffins, mono and di-olefins including cyclopentadiene, piperylenesand isoprene, and smaller amounts of acyclic acetylenes and acetylenicolefins.

When it is desired to recover isoprene, cyclopentadiene or other Chydrocarbons from the main liquid fraction, it is normally firstseparated by fractional distillation into a stream consisting mainly ofC hydrocarbons having an approximate boiling range of to 50 C., and aheavier hydrocarbon stream which may be employed after suitabletreatment in motor gasoline.

The C hydrocarbon stream so obtained may vary considerably incomposition since this is dependent on the cracking conditions employed,the composition of the cracking feedstock, and the subsequent processingof the cracked product. The concentration of the main components in themixture generally lie within the following ranges:

Components: Weight, percent Isoprene 8 to 16. Piperylenes 6 to 14.cyclopentadiene 4 to 25. Isopentane 9 to 20. N-pentane 25 to 45.Cyclopentane 2 to 12. Pentenes 12 to 30. Isopropenyl acetylene 0.02 to0.2. Other acetylenes less than 0.1.

The composition of a C stream from an ethylene pro ducing plantemploying a standard hydrocarbon charge stock may show far lessvariation than has been indicated above.

Many methods have been suggested for the concentration orseparation ofisoprene-and other valuable C hydrocarbons from such cracked Chydrocarbon streams.

The following initial operations are a common feature of many of themethods suggested:

(1) The liquid is treated under pressure at a temperature in the rangeto C., for several hours. The treatment is carried out in a soakingchamber under sufficient pressure to prevent vapourisation. The objectof the treatment is to convert the greater part of the cyclopentadienepresent in the original mixture to the less volatile compounddicyclopentadiene, which is readily separated from isoprene duringsubsequent distillation.

(2) The product after heat treatment is fractionally distilled in one ormore columns of high efliciency at a high reflux ratio. The main objectof this distillation is to separate the mixture into an isoprene richfraction having a very low piperylene content, and a heavier fraction oflow isoprene content and higher piperylene content.

Such treatment is capable of giving an isoprene 'rich fractioncontaining 25 to 40% by weight of isoprene, and 0.1% by weight or lessof piperylenes.

A disadvantage of the treatment described above lies in the relativelyhigh cyclopentadiene content of the isoprene rich fraction, which underthe best conditions is usually in the range 0.5 to 1.0% by weight or 2.0to 4% by weight based on isoprene in the product. cyclopentadiene is aparticularly injurious impurity when the product is required to be usedwithout further purification or concentration as feed to catalyticpolymerisation processes for the production of polyisoprene, e.g., usinglithium butyl as catalyst. Though it may prove possible to reduce thecyclopentadiene content of the isoprene rich fraction to 0.3 wt.percent, it is not practical to reduce it further.

Examination of the subsequent fractional distillation step for therecovery of isoprene in the heat treated feed as an isoprene concentrateof low piperylene content, shows that unconverted cyclopentadienemonomer in the feed is not effectively separated from isoprene in theoverhead fraction unless a large proportion of the isoprene is allowedto be lost in the bottoms product. Under conditions of high isoprenerecovery in the overhead concentrate, the cyclopentadiene content of theoverhead fraction is generally higher than the cyclopentadiene contenteither of the feed to the column or of the bottoms product.

This behaviour of cyclopentadiene in the fractionating column, which isanomalous when considered in relation to the boiling points of isoprene,cyclopentadiene and cisand trans-piperylene, appears to result from thenon-ideal behaviour of the hydrocarbons of different types (cyclicdi-olefin, branched and straight chain di-olefins, olefins, paraffins,etc.). In scientific terms, the activity coeflicient of cyclopentadienein the liquid is significantly higher than the activity coefficienteither of isoprene or of piperylene, these being nearly identical in anyparticular mixture.

It has now been found that in the fractionation of an isoprene andcyclopentadiene containing C fraction such as is referred to above,either before or after the heat treatment, the cyclopentadiene contentof the liquid during the fractionation is at 'a maximum value at anintermediate point in the fractionation column. This maximumcyclopentadiene content may 'be many times (between five and twenty) thecyclopentadiene content of the isoprene concentrate which is removed asoverhead product, and the peak concentration is generally found near andmost commonly slightly above the mid-point in the fracti'onating column.

It has further been found that by withdrawing liquid from the column inthe area of high cyclopentadiene concentration, dimerizing the majorpart of the cyclopentadiene in the withdrawn liquid and recycling theliquid product mixture to an appropriate point in the column, anisoprene concentrate can be obtained as overhead which contains a highproportion (Le. 80 to 96% by weight) of the isoprene in the feedmaterial, but only traces (less than 0.1% by weight and, if necessary,considerably less) of cyclopentadiene and piperylenes.

The invention thus consists in a process for recovering anisoprene-enriched fraction with a relatively low cyclopentadiene contentfrom a hydrocarbon stream containing isoprene and cyclopentadiene andconsisting predominantly of C hydrocarbons, which process comprisesfractionally distilling the hydrocarbon stream to obtain anisoprene-enriched overhead fraction and during the course of thedistillation withdrawing liquid from an intermediate point in thefractionation system at which a relatively high cyclopentadieneconcentration exists, subjecting the' withdrawn liquid to dimerisationconditions for the dimerisation of cyclopentadiene, and returning theresulting liquid product to an intermediate point in the fractionationsystem, which will normally be close to the point from which liquid iswithdrawn.

The feed material will usually contain piperylenes (cisand trans-) aswell as cyclopentadiene and the piperylenes will be separated in thebottoms fraction.

In the dimerisation treatment during which higher polymers ofcyclopentadiene may also be formed, suitable conditions are atemperature of 100 to 160 0, preferably 120 to 140 C., a pressure offrom to 30 atm., preferably to 25 atm., at least sufficient to maintainthe cyclopentadiene in the liquid phase and a contact time of from 10minutes to 6 hours, preferably minutes to 3 hours.

By dimerising a liquid containing an increased amount of cyclopentadieneas compared with the starting material, the rate of dimerisation isincreased and the size of the dimerisation reactor can be reduced. Alsothe possibility of reaction between the cyclopentadiene and isoprene ismuch reduced, thereby largely avoiding an appreciable loss of isoprene.

The process of the invention may be carried out as a continuous, batchor semi-continuous operation in suitably designed equipment.

The feed material to which the process of the invention may be appliedis a hydrocarbon stream containing the hydrocarbons isoprene,cyclopentadiene, and usually cisand trans-piperylenes. Preferably thefeed material is a fraction of relatively narrow boiling range to 50 C.)and consisting predominantly of C hydrocarbons.

If the cyclopentadiene content of the feed material is high or if aproduct with a very low cyclopentadiene content is required, the feedmaterial may first be subjected to a heat treatment for several hours inthe liquid phase at an elevated temperature preferably in the range 100to 140 C., and under pressure as in the known method previouslydescribed. Suitable conditions are a contact time of 3 to 6 hours, atemperature of 125 C., and a pressure of at least 10 atmospheresabsolute, preferably in the range 15 to atmospheres, to preventvapourisation of the material during heat treatment. This pretreatmentis desirable when the feed has more than 10% of cyclopentadiene, and mayadvantageously be employed when the cyclopentadiene content is over 5%.

In cases where the feed material has an excessively high cyclopentadienecontent, it is preferably subjected additionally to a simpledistillation step before the further treatment described below, toseparate the major part of the C hydrocarbons in the feed fromcyclopentadiene polymers formed by the heat treatment, and so lessen thedegree and incidence of fouling in the subsequent processing equipment.When such a simple distillation step is carried out, the maximumtemperature during distillation should not exceed 140 C., and shouldpreferably be below 100 C., to reduce or prevent thermaldepolymerisation of the cyclopentadiene dimers separated from the bulkof the C hydrocarbons during the distillation.

In carrying out the process of the invention, the feed material, with orwithout the pretreatments described above, may be fractionated in anefficient fractional distillation plant or column containing theequivalent of between 60 and 200 theoretical plates-preferably between100 and 150 theoretical plates. The column is provided with a means ofwithdrawing part or all of the liquid refiux passing down the columnfrom an intermediate point in the column which is preferably situatedslightly above the mid tray of the column. The column is also providedwith a point for returning the liquid removed for processing to anintermediate point in the column, which may be above or below the pointfrom which liquid is removed. In a plant designed for continuousoperation, the point of liquid return is preferably located on the plateimmediately below the point from which liquid is removed. In a plantdesigned for batch operation, the point of liquid return is preferablyseveral plates above the point from which liquid is removed. Thematerial is usually returned to a point at which the isoprene/piperylenes ratio substantially corresponds to that of the returnedmaterial.

By subjecting the withdrawn liquid to dimerisation condit-ions for thedimerisation of cyclopentadiene, e.g. in a soaking vessel, and returningthe liquid product to the column, the maximum cyclopentadiene content ofthe liquid in the column may be reduced to 1 wt. percent or less.

A number of embodiments of the invention are described below, by way ofexample, with reference to the accompanying drawings in which:

FIGURE 1 is a flow diagram illustrating the continuous treatment of a Cfraction containing 5 wt. percent or less cyclopentadiene,

FIGURE 2 is a flow diagram illustrating a batch operation according tothis invention, and

FIGURE 3 is a flow diagram illustrating a modified form of batchoperation.

It should be understood that feed streams having a cyclopentadienecontent higher than 5 weight percent are best subjected to thepretreatment steps of heat treatment and in certain cases simpledistillation as described previously to reduce the cyclopentadienecontent of the feed before being treated by the processes describedbelow.

The feed material may also be subjected to a further pretreatment beforeentering the continuous version of the process shown in FIGURE 1. Thispretreatment consists of a topping distillation in a high efiiciencyfractionating column, e.g. equivalent to 40 to 100 theoretical plates.The object of this topping operation is to remove C bydrocarbons andother light ends, and particularly isopropenyl acetylene which is anactive catalyst poison in many polymerisation processes employingisoprene. At the same time, some of the other low boiling C hydrocarbonsin the feed, e.g., isopentane, may be partially removed withoutsignificant loss of isoprene. This results in a higher concentration ofisoprene in the end product from the process.

Referring to FIGURE 1, the isoprene and cyclopentadiene containing feedstream, subjected if necessary or if desired to one or more of thetreatments described above, passes as liquid in line 1 to the heatexchanger 101 where it is preheated to a suitable temperature (30 to C.depending on the column pressure) before entering the fractionatingcolumn 102 at an intermediate point via line 2. The preheater 101 is notessential and may be dispensed with. The fractionating column 102 is ahigh efliciency liquid vapour counter-current contacting device fittedwith a number of trays or filled with a suitable type of packing, inknown fashion. The column efficiency is preferably not less than 60theoretical plates, and desirably not less than theoretical plates.

Part of the liquid passing down to the base of the column leaves by line3 to the reboiler 103 where it is partially vapourised by indirect heatexchange with steam or other convenient heating medium, and returns tothe column by line 4. The rest of the column base product,

substantially free of isoprene, passes via line to pump 104 and thencevia line 6 to heat exchanger 101 in which it is cooled, and then by line7 to storage or further treatment before utilisation, egg. in theproduction of motor gasoline.

Overhead vapour from the column passes via line 8 to the condenser 105,from which condensed hydrocarbon liquid passes via line 9 to the refluxdrum 106. The pressure in the column, reflux drum and other processingequipment is controlled by admitting or withdrawing gas or. vapour to orfrom the reflux drum via line 14a. The pressure in the reflux drum maybe controlled at any suitable pressure from atmospheric to 20atmospheres, but a pressure of 2 to 3 atmospheres absolute is generallyfound most convenient.

, Liquid passes from the reflux drum 106 via line 10 to the pump 107,from which it is discharged by line 11. Part of the pump discharge isreturned as reflux to the top of the column via line 12.

The remainder passes via line 13 to the cooler 108 where it is furthercooled, and thence via line 14 to product storage. This is the isoprenerich concentrate containing only traces of cyclopentadiene andpiperylene which i the final product from the process.

A special liquid draw-off tray 102a is provided in the column, usually afew trays above the feed tray. Part and preferably all of the liquiddescending on to this tray is removed via line 15 and passes to pump109, which raises its pressure by at least 10 atmospheres andpreferably- 15 to 30 atmospheres. Liquid passes from the pump via line16 to the heat exchanger 110 where it is preheated to a temperature inthe range 70 to 120 C. The liquid leaves the heat exchanger via line 17,passing to the additional heater 111, where it is heated by indirectheat exchange with steam or other suitable medium to a' temperature inthe range 100 to 150 C.

The heated liquid passes via line 18 into the soaking vessel 112, whichis preferably provided with internal baffles to reduce back mixing andshort circuiting within the vessel. The vessel is freed of gas or airbefore start up. The volume of the vessel 112 is so arranged as toproduce a liquid residence time in the vessel in the range 10 minutes to6 hours, preferably between minutes and 3 hours, depending on thetemperature in the vessel and the degree of cyclopentadiene removalrequired. Liquid leaving the soaking vessel 112 via line 19 is cooled byheat exchange with incoming liquid in 110 and returned to the column,preferably to the tray below the liquid draw-off, by line 20. When allliquid is withdrawn from tray 102a, the returning liquid shoulddesirably enter the tray below the tray 102a. Line 20 is provided with athrottling or back-pressure control valve 20a, preferably of theautomatic type, to prevent loss of pressure and vapourisation in thesoaking vessel 112.

Preheater 111 may be by-passed or dispensed with when the amount ofcyclopentadiene in the feed is high, since the dimerisat-ion ofcyclopentadiene occurring in 112 raises the temperature of the liquidand so causes an adequate temperature difference across the heattransfer surface of the heat exchanger 110.

FIGURE 2, shows a flow diagram of the process in which the invention isapplied in a batch manner. This has advantages over continuousprocessing where the scale of operation 'is relatively small,particularly in so far as it The batch charge in the kettle 201 isheated and vapour"- ised in the normal manner, vapour passing throughthe high efliciency fractionating column 202, leaving v-ia line 22 andpassing to the condenser 203. Condensed liquid passes from the condenservia line 23 to the reflux drum 204, which is preferably designed to havea very low liquid capacity in relation to the capacity of the kettle201. The pressure in the equipment described is controlled by admittingor withdrawing gas or vapour to or from the reflux drum via line 40.

Condensed liquid passes from the reflux drum 204 via line 24 to thereflux/product pump 205, from which it is discharged via line 25. Themajor part of this stream returns as reflux to the top of the column 202via line 26, whilst the smaller part passes as product via line 27 tothe cooler 206, which it leaves vialine 28.

In the batch process, several other overhead fractions may be separatedfrom the charge in addition to the isoprene rich concentrate, thuseliminating the need for additional equipment generally required inconjunction with the continuous process. Product line 28 is connected tobranch lines 29, 30, 31 and 32 for removal of the various overheadfractions.

Line 29 is used for removal of the first fore-running fraction whichconsists of C hydrocarbons, isopropenyl acetylene, isopentane and otherlow boiling C hydrocarbons, but is substantially free of isoprene. Thisfraction may be employed in the manufacture of motor gasolene. Line 30is used for removal of the isoprene rich concen trate, substantiallyfree from cyclopentadiene, piperylenes and isopropenyl acetylene. Thisis the main product from the process. Line 31 may be used for removingan intermediate fraction containing both isoprene and piperylenes afterthe main product has been removed.

This intermediate fraction may advantageously be added to the charge ofa subsequent batch and so be reprocessed.

Line 32 is used for withdrawing a heavy C distillate fraction containingpiperylenes, pentenes, normal and cyclopentane and higher boilinghydrocarbons. This fraction, which is substantially free ofcyclopentadiene and dicyclopentadiene, may be used in the manufacture ofmotor gasoline with the minimum of further treatment.

Part of the liquid descending the fractionating column 202 is withdrawnfrom a specially provided draw-off tray 202a via line 33 to pump 207,which raises its pressure to at least 10 atmospheres and preferably to15 to 30 atmospheres. The liquid passes via line 34 to the heatexchanger 208 where it is heated to a temperature in the range 70 to C.and thence via line 35 to auxiliary heater 209 which raises thetemperature of the liquid further to 100 to C. before entering thesoaking vessel 210, via line 36, provided with internal baflles as inthe continuous process and designed to give a liquid residence timewhich is preferably within the range of 30 minutes to 3 hours. Liquidleaving the soaking vessel via line 37 is cooled by heat exchange withincoming liquid in heat exchanger 208 and returned to the column vialine 38. This line is provided with a throttling valve 380, preferablyof the automatic type, to prevent loss of pressure and liquidvapourisation in the soaking vessel 210-.

The point at which the liquid is returned to the column is in the caseof the batch process preferably several plates higher in the column thanthe point of liquid offtake. The reason for this is that the compositionof the liquid at any point in the column is continuously changing duringbatch distillation, so that by the time the liquid leaving the take-offtray has passed through the time tank, the concentrations'of isopreneand piperylenes in the liquid correspond to those of a point in thecolumn higher than the point from which the liquid has been removed fromtreatment.

The liquid charge introduced into the kettle for batch operation shouldbe large in relation to the capacity of the soaking vessel 210, and theperiod of the total batch cycle should be at least 24 hours, preferably72 hours or more for eflicient operation of the process described.

The operation is started at total reflux and sufficient time is givenfor the concentration gradient to be established in the column beforeoff-take of fore-runnings via line 29 is commenced. The soaking vessel210 is conveniently left full of liquid from the previous batch, andflow of liquid through the soaking vessel is commenced as soon as refluxliquid is descending the column. The reflux ratio is preferablyincreased progressively during withdrawal of each fraction, but loweredon commencement of a fresh fraction.

The liquid left in the kettle at the end of the distillation willcontain 95% or more of the cyclopentadiene in the feed in the form ofdimer and higher polymers and co-polymers. It is not advisable toattempt to remove all heavy C hydrocarbons from the kettle by raisingthe temperature too far at the end of the batch distillation, since thiscan cause depolymerisation of di-cyclopentadiene in the kettle. Thekettle temperature at the end of the distillation should not exceed 150C., and should preferably not exceed 100 C.

Liquid remaining in the kettle at the end of the distillation is removedvia line 39 by pump 212, by which it is pumped to separate storage. Thisliquid may be utilised for the manufacture of cyclopentadiene and itsderivatives.

Control valves 29a, 30a, 31a and 32:; are suitably provided in the lines29, 30, 31 and 32 respectively. Valve 38a, a back pressure valve similarto valve 20a of FIG- URE 1, is provided in line 38.

As previously stated, a modified batch process which may be referred toas a semi-continuous process, is illustrated in FIGURE 3. In place ofthe kettle in the batch process, the charge is contained in a tank orsuitably constructed vessel 301 which is maintained at a lowertemperature than that in the kettle for the batch process. The charge isfed to vessel 301 by line 41 and is circulated at a high rate via line42 by pump 302, thence via line 43 through heat exchanger 303, thencevia line 44 through the vapouriser 304 and finally via line 45 into thebase of the column 305 above the liquid level in the column. Liquid isremoved from the base of the column 305 via line 46 by pump 306, thencevia line 47 to heat exchanger 303, thence via line 48 to cooler 307, andback into the charge vessel 301. The rest of the operation and equipmentis as described for the batch process, the same reference numerals beingused to designate the rest of the equipment as are used in FIGURE 2,with the exception that the plate from which liquid is withdrawn isdesignated 305a. Liquid left in the charge vessel 301 at the end of thedistillation is pumped away via line 50.

The main advantage of the semicontinuous process is that it enables verylarge batches of material to 'be processed over a long batch cycle,(i.e. a week or more) without subjecting the material to prolongedresidence times at the kettle temperature, and consequent loss ofisoprene and fouling, which would be a disadvantage of the batch processunder similar circumstances.

The semi-continuous process further enables isoprene in the feed to berecovered in the product concentrate at yields in excess of 95%, with acyclopentadiene content of less than 0.05 wt. percent.

The fractionation column is operated at a high reflux ratio. In the caseof continuous distillation, the preferred reflux: feed ratio lies withinthe range :1 and :1. In the case of batch distillation the reflux ratiois preferably increased continuously as distillation of the isoprenerich concentrate proceeds, e.g., from an initial reflux: product ratioof 5:1 to a final reflux: product ratio of 1.

Whilst a single column has been shown in the processes described above,it should be understood that this may consist of two or more columnsplaced in series with intermediate vapour lines, liquid transfer linesand pumps. Such an arrangement is frequently used where a large numberof theoretical plates are required.

The process of the invention and the advantage obtained thereby isillustrated by the following example.

EXAMPLE A high efliciency continuous pilot plant fractionating columnequipped with special gauze packing was set up with ancillaries as shownin FIGURE 1. An internal overflow weir allowed all liquid to fiow downthe column when the soaking vessel (112) was not in use and no liquidwas being withdrawn. The efllciency of the column was tested at a totalreflux :at a top pressure of 15 p.s.i.g. using a mixture of isoandnormal pentanes as test mixture. Analyses of samples taken from thereflux drum and column base showed that the column had an averageefficiency of 78 theoretical plates.

The feed point to the column corresponded to 36 theoretical plates abovethe reboiler. The liquid draw-off tray was located immediately above thefeed point, and the liquid from the soaking vessel was returned to thefeed point in the column.

The column was fed with 65 pounds. per hour of a cracked C hydrocarbonfeed at 50 C. from which part of the cyclopentadiene had been removed byheat treatment and distillation.

The composition of the treated feed, analysed by gasliquidchromatography was as follows:

Weight percent Isopentane 13.4 n-Pentane 22.6 2-methyl butene-l 9.4Pentene-l 8.6 Pentene-2 (cisand trans-) 7.3 2-methyl butene-2 3.8Isoprene 10.6 Piperylenes (cisand trans-) 11.5 Cyciopentane 11.9cyclopentadiene 0.7 Unidentified 0.2

The column was operated at a top pressure of 15 p.s.i.g.

and a top temperature of 49 C. The liquid boil up rate and reflux returnwere adjusted to give an internal reflux:feed ratio of 16:1, i.e., aninternal reflux rate of 1040 pounds per hour. The soaking vessel 112 hada capacity of 20 cubic feet, corresponding to a liquid residence time ofapproximately 40 minutes, and was operated at a temperature of C. and apressure of 290 p.s.i.g.

The column was first operated with the soaking vessel out of commission,valves on line 15 and 20 being closed, and all liquid on the take-offtray overflowing down the column. a

After careful adjustment of the temperature and flow rates, there wereobtained 26 pounds/hour of overhead product from line 14 containing25.5%. isoprene, 0.2% piperylenes and 1.1% cyclopentadiene.

The bottoms product from line 7 contained 0.5% isoprene, 18.7%piperylenes and 0.4% cyclopentadiene as monomer.

The column continued to operate on the same feed and at the same feedrate as before: the valves on lines 15 and 20 were opened, and the pump,heat exchangers and soaking vessel were brought gradually intocommission. A period of 28 hours was allowed to reach steady conditionsfrom the time the soaking vessel was full of liquid.

25 pounds per hour of overhead product were withdrawn from line 14,containing 26.4% isoprene, 0.15% piperylenes and 0.08% cyclopentadiene.

The bottoms product from line 7 contained 0.4% isoprene, 18.2%piperylenes and 0.05% of cyclopentadiene as monomer.

These runs showed that operation inaccordance with the invention ascompared with a normal fractional dis tillation resulted in a 14-foldreduction in the cyclopentadiene content of the overhead product withoutloss of isoprene.

All percentages referred to in the example are percentages by weight.

We claim:

1. A process for the recovery of an isoprene fraction with a very lowcyclopentadiene content as an overhead fraction by fractionaldistillation from a hydrocarbon stream containing isoprene andcyclopentadiene and consisting predominantly of C hydrocarbons, in whichprocess liquid is withdrawn during the course of the fractionaldistillation from an intermediate point in the fractional distillationsystem in an area of relatively high cyclopentadiene concentration, thewithdrawn liquid is subjected to dimerisation conditions for thedimerisation of cyclopentadiene and the resulting liquid product isreturned to an intermediate point in the fractional distillation system.

2. A process as claimed in claim 1, in which the hydrocarbon streamcontains piperylenes and in which an isoprene-enriched fraction isrecovered as overheads and a piperylene-enriched fraction is alsorecovered, the isoprene-enriched fraction having a considerably reducedcyclopentadiene and piperylenes content.

3. A process according to claim 2 in which the said resulting liquidproduct is returned to a point in the fractional distillation system atwhich the isoprene/piperylene ratio corresponds approximately with thatof the said resulting liquid product.

4. A process as claimed in claim 1, in which the hydrocarbon streamcontains more than by weight of cyclopentadiene and is subjected to apreliminary treatment for the dimerisation of cyclopentadiene to reducethe cyclopentadiene content below 5% by weight.

5. A process as claimed in claim 4, in which the hydrocarbon streamcontains more than 5% by weight of cyclopentadiene and is subjected tothe said preliminary treatment.

6. A process as claimed in claim 4, in which the hydrocarbon streamafter said preliminary treatment is distilled to separate the polymersformed and then fractionally distilled to recover the overhead isoprenefraction.

7. A process as claimed in claim 1, in which the hydrocarbon stream issubjected to a preliminary distillation to obtain a stream of increasedisoprene content, which is subjected to said fractional distillation.

8. A process as claimed in claim 1, in which the withdrawn liquid isheated to a temperature of from to 160 C., under a pressure of 10 to 30atmospheres sufficient to maintain the cyclopentadiene in the liquidphase and for a period of from 10 minutes to 6 hours.

9. A process as claimed in claim 8, in which the withdrawn liquid isheated to a temperature of from to C., under a pressure of from 15 to 25atmospheres and for a period of from 20 minutes to 3 hours.

10. A process for the recovery of an isoprene fraction with aconsiderably reduced cyclopentadiene and piperylenes content from ahydrocarbon stream containing isoprene, piperylenes and more than 5% byweight of cyclopentadiene and consisting substantially completely of Chydrocarbons, which process comprises subjecting the hydrocarbon streamto a preliminary treatment for dimerisation of cyclopentadiene to reducethe cyclopentadiene content below 5% by weight, distilling thehydrocarbon stream to separate the polymers formed therefrom,fractionally distilling the hydrocarbon stream from which the polymershave been separated, withdrawing during the course of the fractionaldistillation liquid from an intermediate point in the fractionaldistillation system in an area of relatively high cyclopentadieneconcentration, subjecting the withdrawn liquid to dimerisationconditions for the dimerisation of cyclopentadiene, returning theresulting liquid product to an intermediate point in the fractionaldistillation system, and recovering an isopreneenriched fraction asoverheads product and a piperylenesenriched fraction as bottoms product.

11. A process as claimed in claim 10, in which the withdrawn liquid isheated to a temperature of from 120 to 140 C., under a pressure of from15 to 25 atmospheres and for a period of from 20 minutes to 3 hours.

References Cited by the Examiner UNITED STATES PATENTS 2,704,778 3/1955Maisel 260-681.5 2,768,224 10/1956 Page et al 260-6815 DELBERT E. GANTZ,Primary Examiner.

G. E. SCHMITKONS,Assistant Examiner.

1. A PROCESS FOR THE RECOVERY OF AN ISOPRENE FRACTION WITH A VERY LOWCYCLOPENTADIENE CONTENT AS AN OVERHEAD FRACTION BY FRACTIONALDISTILLATION FROM A HYDROCARBON STREAM CONTAINING ISOPRENED ANDCYCLOPENTADIENE AND CONSISTING PREDOMINANTLY OF C5 HYDROCARBONS IN WHICHPROCESS LIQUID IS WITHDRAWM DURING THE COURSE OF THE FRACTIONALDISTILLATION FROM AN INTERMEDIATE POINT IN THE FRACTIONAL DISTILLATIONSYSTEM IN AN AREA OF RELATIVELY HIGH CYCLOPENTADIENE CONCENTRATION, THEWITHDRAWN LIQUID IS SUBJECTED TO DIMERISATION CONDITIONS FOR THEDIMERISATION OF CYCLOPENTADIENE AND THE RESULTING LIQUID PRODUCT ISRETURNED TO AN INTERMIEDCIATE POINT IN THE FRACTIONAL DISTILLATIONSYSTEM.